Global navigational satellite systems (GNSS) exist in which mobile terrestrial satellite receivers may calculate their position based on data received from satellites. The terrestrial satellite receiver receives information relating to a satellite position broadcast from the satellite. This information may include information describing an orbit of the satellite and timing information for the satellite. The terrestrial satellite receiver calculates its position using this information. Examples of GNSS systems are the Global position system GPS, Galileo, GloNass and Compass.
Recently GNSS receivers have been introduced which are able to provide their positional data to other entities. These receivers typically have a modem embedded in the receiver capable of communicating using cellular (GPRS), radio or satellite links. These receivers have application in the security field in that they enable the whereabouts of a vehicle to be tracked. They are also applicable to the emergency services field where they may be used to transmit mayday and positional data. More recently the use of GNSS receivers in so-called electronic fee collection systems for example toll fees, road tax and congestion charges has been suggested.
The transmitted positional data signal should be robust. For example, a disruption of the positional data signal may prevent a stolen vehicle from being tracked. Additionally, the payment of road tax, toll fees and congestion charges may be avoided by interfering with the positional data signal.